Designing a Planetary Wheel (Almost)

[sanddrag]

Someone else tried to desing one a little while back:
http://www.chiefdelphi.com/forums/sh...lanetary+wheel
http://www.chiefdelphi.com/forums/sh...lanetary+wheel

and for the most part it was determined that there are a lot of inherent design flaws and it would take an eternity to get the bugs worked out. I'm not saying it won't work, but if you decide to pursue it, be sure you are into it for the long haul. Planetaries can be tricky little things buy themselves, and put all the loads associated with a wheel on one and then you are really asking for it. There are better ways to accomplish the same task.

But don't let me stop you. Who knows, you might discover a big breakthrough in a durable, easy to build planetary wheel.

Oh yes, and there is the "edit" button so you needn't double post.

[Henry_Mareck]

I don't know if I am missing something obvious, but i cant think of what the advantages of having a planetary wheel would be.
Can someone explain them?

[patTeam241]

I hope that this adds something to the discussion. I've played with(but never extensively) planetary gear systems . However, I don't understand exactly where the benefit is, could someone explain this? Is it just that it keeps it the outer gear centered? Does it improve performance? Thanks. (This should apply to either the gear or wheel) (Guess this kinda goes w/ the previous post we entered them at the same time)

Chain weighs quite a bit, so one point of goodness is weight. Another is that it takes up much less space than a "conventional" drive train. I can't personally think of any more positives, but I'm sure there are more...

[sanddrag]

The main benefit is lots of reduction in a relatively small space while keeping the output shaft on the same axis as the input. Take for example the globe motor. Do you have any idea how large of a reduction is in that tiny planetary gearhead? Something like 115:1. Try to do that with spurs in that amount of space.

[Veselin Kolev]

Originally Posted by sanddrag

The main benefit is lots of reduction in a relatively small space while keeping the output shaft on the same axis as the input. Take for example the globe motor. Do you have any idea how large of a reduction is in that tiny planetary gearhead? Something like 115:1. Try to do that with spurs in that amount of space.

I did something similar in 2004 with swerve drive, having the gear reduction inside the wheel. But I have to ask, why planetary? It costs much less to just use external gears, instead of internal gears. The only thing you dont get anymore with one reduction is the axis of the drive shaft is no longer on the same axis as the wheel. So just use two reductions. I did in the swerve I designed, and it was very spiffy. In fact, people were asking if it had a planetary reduction in it, but it didnt. It was very similar, used a lot less gears, and a lot less bearings. With a reduction of 1:42.

If you look at the link to the concept sanddrag posted, you can tell that the gear reduction in the planetary is just the teeth of the sun gear over the teeth of the internal gear. Why not just make the external gear an internal gear? Solves a lot of problems. But hey, I can understand if you're doing it beacuse its awesome. It is. Having planetary gears reduces the stress on all the gears because more teeth are in contact at the same time. It's a cool idea, I just want you to be aware that it is more costly than several very similar alternatives.

[sanddrag]

Originally Posted by Veselin Kolev

If you look at the link to the concept sanddrag posted, you can tell that the gear reduction in the planetary is just the teeth of the sun gear over the teeth of the internal gear. Why not just make the external gear an internal gear?

Isn't it sun/(sun+ring) in a fixed ring planetary?

[ConKbot of Doom]

Originally Posted by sanddrag

Isn't it sun/(sun+ring) in a fixed ring planetary?

On the linked exploded image, the planet gears were fixed, the ring was the wheel and the sun was the input.

[lndnny]

Thanks for all of these ideas! What I ment when I when I said planetary was what Veselin Kolev was thinking. I'm trying to design a swerve drive and was tight on space and weight. I figured out the problem, two "circular plates" were supposed to be spinning at the same speed, but one was spinning slightly slower. Nothing a little rearranging couldn't fix

Thanks

Team 808
Dan Lin

[calieanZET]

Originally Posted by patTeam241

...However, I don't understand exactly where the benefit is, could someone explain this? ...

As far as the benefits that planetary gears provide there are several:

1. The loading of the gears is reduced so smaller gears (lighter weight) can be used for the same torque ratings. Instead of having the power transmitted on one side of a gear to one side of another gear, the power is transmitted from several points, reducing the forces at any one point.

2. The overall effectiency of the gear system is greater then chains or belts.

3. The most useful benefit of planetary gears is planetary gears can also provide multiple gear ratios. Depending on which gear (or set of gears) is stationary you get different output speeds. for instance if you have the sun gear as the input, keep the planet gears stationary and the rotation of the ring gear is the output you get one speed. if you hold the ring gear stationary and allow the planet gears to rotate about the sun gear, and use that as the output, you get a different output. If you feed the output of one planetary set, into another planetary set you can have 4 different gear ratios. This method of changing gear ratios has the potential to save weight compared to the more conventional methods because there are no "extra" gears. Most of the multispeed transmissions i have seen at first swap gears in the transmission so that there is always atleast one gear that is not in use, and one that is extra weight not included in the planetary gear system.

4. Planetary gears sets also have the potential to take up less space, then conventional transmissions.

In summary planetary gears do the following:
1. reduce weight
2. increase efficiency
3. able to have multiple speeds
4. decrease size

[RogerR]

Originally Posted by calieanZET

...2. The overall effectiency of the gear system is greater then chains or belts...

its been discussed on this message board, and the general consensus among the experts is that a single planetary stage's efficiency is usually 90% or less. and while belts usually have a lower efficiency, a properly tensioned and aligned chain can have an efficiency of (up to) 98%.

[patTeam241]

Thanks alot.